Last week, a new Stanford study answered one of the oldest questions in autism research: Does autism differ between boys and girls?

Yes, the study found. According to the results published in Molecular Autism, girls tend to have less severe manifestations of one core feature of autism, repetitive and restricted behavior, and they show brain-scan differences from boys that help explain the discrepancy.

Hints of a gender difference in autism extend all the way back to the first clinical report of the disorder, published in 1943, which described eight boys and three girls. 70 years after that report, boys are still more commonly diagnosed; among kids with high-functioning autism, the ratio of boys to girls is four to one. But questions about the nature of the gender split have persisted, as our press release on the work explains:

“We wanted to know which specific clinical manifestations of autism show significant gender differences, and whether patterns in the brain’s gray matter could explain behavioral differences,” said the study’s senior author, Vinod Menon, PhD, professor of psychiatry and behavioral sciences. Knowledge of the difference could help clinicians better recognize and treat autism in both sexes, he added. “Understanding this is really quite crucial clinically.”

Prior studies of the gender difference have typically been small and inconclusive. So the Stanford team took a new approach, using two large public databases that capture information from patients all over the country. This allowed them to study almost 800 children who fit a fairly narrow set of criteria: age 7 to 13, evaluated by standard tests for autistic behavior, and with an IQ above 70.

Girls had lower (more normal) scores than boys on a standard evaluation of repetitive and restricted behavior, which includes preoccupation with narrow interests, inflexibility about routines and repetitive motions such as hand-flapping, the study found. Girls and boys did not differ on measures of autism's other core features, which are social and communication deficits.

MRI brain scans on a subset of the children showed different grey-matter patterns between boys and girls in the motor cortex, supplementary motor area and part of the cerebellum. These areas help the brain plan and carry out motor functions, which the researchers said was noteworthy because many repetitive behaviors have a motor component.

“The discovery of gender differences in both behavioral and brain measures suggests that clinicians may want to focus diagnosis and treatments for autistic girls differently than boys,” [lead author Kaustubh] Supekar,PhD, added.

The scenario many of us learned in school is that two X chromosomes make someone female, and an X and a Y chromosome make someone male. These are simplistic ways of thinking about what is scientifically very complex.